1. Field of the Invention
The present invention relates to an injection molding machine equipped with a drive mechanism which reciprocates (i.e., advances and retreats) a movable member, such as a screw or an input member of a mold clamping mechanism, by use of a servomotor and a ball screw mechanism.
2. Description of the Relevant Art
Japanese Patent Publication (kokoku) No. 8(1996)-2567 and Japanese Patent Application Laid-Open (kokai) No. 10(1998)-151653 disclose a conventional injection molding machine in which a screw is reciprocated by means of a drive mechanism including a servomotor and a ball screw mechanism.
In general, an injection molding machine of such a type is designed such that the rear end of the screw is coupled to the nut of the ball screw mechanism; a servomotor is disposed on one side of the machine body; and rotation of the servomotor is transmitted to the ball screw of the ball screw mechanism via a rotation transmission mechanism. Thus, the rotational motion transmitted from the servomotor is converted to a linear motion by the ball screw mechanism; and the screw is advanced and retracted as a result of the linear motion of the nut. The rotation transmission mechanism consists of a toothed drive pulley attached to a rotor shaft of the servomotor, a toothed driven pulley attached to the ball screw of the ball screw mechanism, and a timing belt looped around and extending between the toothed drive pulley and the toothed driven pulley.
The above-described conventional injection molding machine has the following drawbacks.
First, the necessity of a space for disposing the rotation transmission mechanism results in an increased size of the machine body of the injection molding machine. In addition, the rotation transmission mechanism generates noise and decreases the rigidity. In particular, the decreased rigidity makes it difficult to increase the control gain (control constant), resulting in unstable control.
Second, since rotation is transmitted to the ball screw via the rotation transmission mechanism, responsiveness of control and accuracy of control decrease, and unbalanced load is imposed on the rotor shaft of the servomotor from the timing belt.
An object of the present invention is to provide an injection molding machine capable of reducing the size of the machine body and noise and increasing the gain (control constant) of a control system to thereby enhance the stability of the control system.
Another object of the present invention is to provide an injection molding machine capable of drastically enhancing responsiveness of control and accuracy of control and avoiding imposition of unbalanced load on the rotor shaft of a servomotor which would otherwise result from use of a timing belt.
To achieve the above objects, the present invention provides an injection molding machine equipped with a drive mechanism which reciprocates a movable member, such as a screw or an input member of a mold clamping mechanism, by use of a servomotor and a ball screw mechanism. In the injection molding machine, an end portion of a rotor shaft of the servomotor is connected directly to an end portion of a ball screw (or a nut) of the ball screw mechanism for reciprocating the movable member. This structure enables rotation of the servomotor to be transmitted directly to the ball screw (or the nut) of the ball screw mechanism without intervention of a rotation transmission mechanism.